Monthly Archives: December 2016

Pictured above is the internal mold of an ammonoid fossil – a group of invertebrate marine animals abundant in the world’s oceans from 416 – 66 million years ago. They died during the same mass extinction that killed the dinosaurs.

Ammonoids were not stationary bottom dwellers, but had an interesting way of getting around in the water. Their shells were partitioned into chambers, which are evident in the picture above. The squid-like ammonite only occupied the final chamber of the shell. The rest were empty so that the animal could control its buoyancy, and swim by taking in and expelling water.

Because ammonoids were abundant, widespread, and evolved new species quickly, geologists use their fossils to correlate rock units of similar age worldwide. This one was collected from the Fayetteville Shale in northwest Arkansas. Its gold color is due to the original organic material having been replaced by pyrite – also known as fool’s gold.

One of the most fossiliferous formations in the state is the Pitkin Limestone. It was referred to as the Archimedes Limestone in the late 1890s because it contains an abundance of the screw-shaped bryozoan fossil Archimedes. It was formally named the Pitkin Limestone in 1904 for exposures near Pitkin Post Office in Washington County, Arkansas. If you can’t find the town of Pitkin on a map, don’t worry–it’s now known as Woolsey.

The Pitkin began as carbonate sediments deposited in the Mississippian Period around 320 million years ago. At that time, northern Arkansas was covered by a shallow sea that was fairly close to the equator. Warm, shallow seawater is a prime environment for the build-up of carbonates. Marine organisms extracted calcium carbonate out of the seawater to form shells or other hard parts. This material accumulated and eventually turned into limestone. Some of those secreted structures are preserved as fossils in the rock and are clues to the environmental conditions that existed at the time.

The area of what is now Arkansas during the Mississippian

The Pitkin Limestone is a bluff-former that crops out in the southern portion of the Ozark Plateaus from just south of Fayetteville eastward to Batesville, typically along the Boston Mountains Plateau Escarpment. It is mostly limestone, however, there is some nodular black chert present locally. Black shale intervals are common in the eastern portion. Because limestone is a soluble rock, karst features such as caves, sinkholes, springs, and disappearing streams are common in this Formation. About 9% of the known caves in Arkansas are in the Pitkin. Its thickness varies from an average of about 50 feet on the west side of the state to about 200 feet in the eastern part with a maximum of about 400 feet in the central portion. It typically rests on the Fayetteville Shale and is overlain by the Cane Hill Member of the Hale Formation in western Arkansas and by the Imo interval from the area of western Searcy County eastward.

The Pitkin outcrop belt is within the light-brown area in this Ozark Plateaus detail of the Geologic Map of Arkansas

The photos above contain fossils of Archimedes. The fossil is named for the ancient Greek engineer who invented a device that incorporated a large screw to lift water for irrigation. The left photo was taken south of Batesville and the right photo was taken south of Fayetteville. It’s remarkable that these fossils are so persistent along this great extent. Although this fossil is characteristic of the Pitkin, it can also be present in adjacent formations. The illustration below is a sketch of a fenestrate Bryzoan of which Archimedes is a type.

Archimedes as it may have appeared in life

Pieces of fossilized Crinoids are also abundant in the Pitkin. Most commonly, small button-shaped pieces of the stem and arms, known as columnals, are preserved in the limestone. That is a columnal in the center of the left photo. The larger crinoid fossils above were preserved in shale and were most probably washed onto a mud flat during a storm event. These photos were taken south of Batesville, but crinoid detritus is abundant throughout the Pitkin and most other limestone in Arkansas.

Crinoid as it may have appeared in life

A great location to see the Pitkin is along Richland Creek at its confluence with Falling Water Creek. When the creek level is low, you can hike upstream from the campground and see many fine exposures of Pitkin Limestone in the creekbed. Locally, colonies of tabulate and rugose coral were preserved in the Pitkin and can be discovered upon close inspection of the outcrop.

Tabulate or colonial coral in the Pitkin Limestone along Richland Creek.

Rugose coral in Pitkin

Locally, the Pitkin consists of oolite, a type of sedimentary rock composed of ooliths. Ooliths are small, spherical structures (<2 mm) that form by accretion of numerous concentric layers of calcite on a central nucleus such as a shell fragment or sand grain. The environment of deposition would have been areas where strong bottom currents or wave action rolled the fragment around in carbonate-rich sea water. This would include environments like beaches and tidal flats.

Oncolites and stromatolites are also preserved in the Pitkin. They have a similar structure to ooliths, but are much larger (up to 10 cm), can be round or irregular-shaped, and are formed by a different mechanism. Like ooliths, they nucleate on a shell or other fragment, but are built up by encrusting layers of blue-green algae or cyanobacteria. Stromatolites form in much the same way, but create columns, mats, or large heads. Stromatolites and oncolites typically indicate a paleoenvironment of warm, shallow water in a calm sea, lagoon, or bay.

Oolitic Pitkin

Oncolitic Pitkin

Stromatolitic Pitkin

During fieldwork for our geologic mapping, finding Pitkin Limestone is always exciting because there is something new and interesting to discover every time. We hope this brief introduction to one of Arkansas’ most intriguing formations has convinced you to seek out the Pitkin and have a closer look.